The present invention relates to an abrasive-bladed or, in particular, to a diamond-bladed multiple cutting wheel assembly. More particularly, the invention relates to an assembly of a plurality of cutting wheels each bladed on the outer periphery of a base wheel with abrasive particles such as diamond particles and held coaxially on a single shaft by keeping an appropriate space from the adjacent cutting wheels so as to prepare a plurality of machine-cut pieces from a single base block of a hard and brittle material such as sintered magnets of a rare earth-based alloy at a single run of the cutting work.
In an industrial process for the preparation of a large number of, e.g., sintered rare earth-based magnet pieces of the same dimensions, it is alternative that the magnet pieces are prepared one by one in a powder metallurgical process including the steps of powder molding into a green powder compact and sintering of the green body or that a large magnet block is prepared by the powder metallurgical method and the large block is subsequently sliced or cut by using a suitable cutting tool into pieces by taking into account the axis of easy magnetization of the magnet when the sintered magnet is anisotropically magnetic.
In the former process of individual production of magnet pieces one by one repeating a unit powder metallurgical process for a single product, the only requirement for the process is that the single product by the unit process is acceptable and good reproducibility of the process can be ensured with high productivity so that the mechanical working of the sintered pieces can be relatively simple and easy although such an individual process is not suitable for the manufacture of small magnet pieces or magnet pieces of a small thickness in the direction of magnetization because deformation of small pieces or warping of a thin sheet is unavoidably very large relative to the planar dimensions sometimes not to give any acceptable products. These problems are no longer of great significance in the latter process of machining of a large magnet block in which the process control can be undertaken relatively simply for the powder molding and sintering steps so that this process is of the major current of the rare earth magnet production due to the high productivity and good versatility. On the other hand, what is important in this process is the productivity of the machining step of the large sintered magnet blocks obtained by sintering with dimensional accuracy as high as possible and with a material loss as small as possible.
The most widely employed cutting tools for this purpose are abrasive-bladed cutting wheels which can be either of the internal-bladed type or outer-bladed type. An internal-bladed cutting wheel bladed with abrasive particles is an integral body formed from a thin annular base wheel provided with a cutting blade of abrasive particles on and along the inner periphery of the annular base wheel. An outer-bladed cutting wheel is, as is illustrated in FIGS. 1A, 1B and 1C, an integral body 6 formed from a thin disk 1 as the base wheel having a shaft opening 5 and provided with a cutting blade 2 of abrasive particles on and along the outer periphery of the base wheel 1. As a trend in recent years, the outer-bladed cutting wheels are preferred to the internalbladed cutting wheels. The outer-bladed cutting wheels are still more advantageous with a multiplied productivity of the cutting works when a plurality of such cutting wheels 6 are, as is illustrated in FIGS. 2A and 2B by a perspective view and an axial cross sectional view, respectively, assembled coaxially on a single shaft 10 with spacers 3 each between two adjacent cutting wheels 6 enabling to produce a plurality of magnet pieces as cut or sliced in a single run of the cutting works.
The above mentioned abrasive particles are preferably particles of diamond or cubic boron nitride when the material of the workpiece is a sintered rare earth-based magnet alloy. The abrasive particles are bonded or cemented together to form the cutting blade 2 on the outer periphery of the base wheel 1 by the resin-bond method using a resinous bonding agent, metal-bond method using a metallic cementing agent or electrodeposition method forming a metallic plating layer on the surface of the abrasive particles, of which the resin-bonded cutting blades are preferred in the cutting works of a rare earth-based magnets or, in particular, sintered magnets of a rare earth-iron-boron alloy as a high-hardness material. This is because the holding strength on the resin-bonded abrasive particles is less firm due to the lower mechanical strengths of the resinous bonding agent as compared with metallic cementing agents but the low elastic modulus of the resinous bonding agent ensures a soft-touch condition between the workpiece and the abrasive particles and lastingly excellent quality of cutting. In a metal-bonded cutting wheel of abrasive particles, namely, the particle-holding strength and the wearing resistance of the cutting blade layer can be high due to the high mechanical strengths and high elastic modulus of the metallic cementing agent while these advantages are accompanied by a disadvantage that the abrasive surface of the cutting blade layer readily becomes dull to exhibit an increased cutting resistance although cutting wheels with a metal-bonded abrasive blade are also used in the cutting works of rare earth-iron-boron sintered magnet blocks, though not so widely practiced.
In conducting multiple cutting of a large rare earth magnet block by using a multiple cutting wheel assembly illustrated in FIGS. 2A and 2B to obtain a plurality of magnet pieces as cut or sliced in a single cutting run, one of the most important factors to be taken into consideration besides the accuracy of the cutting work is the relationship between the thickness of the abrasive-bladed cutting wheels and the material loss of the sintered rare earth magnet because a larger thickness of the abrasive-bladed cutting wheels necessarily results in an increase in the material loss by cutting and a decrease in the number of magnet piece products leading to a decrease in the productivity and increase of the production costs.
When the thickness of the abrasive cutting blade 2 is desired to be decreased in order to decrease the material loss of the rare earth magnet, it is of course that the thickness of the base wheel 1 must also be as small as possible while thickness of the base wheel 1 is limited depending on the material thereof. In view of the low costs and high mechanical strengths, the material of base wheels in conventional abrasive-bladed cutting wheels is almost exclusively limited to alloy tool steels including the grades of SK, SKD, SKT and SKH specified in JIS (Japanese Industrial Standard). When an abrasive-bladed cutting wheel with a steel-made base wheel is employed in the cutting work of a very hard material such as sintered rare earth magnets, however, the mechanical strength of the base wheel is still insufficient so that troubles are sometimes encountered such as distortion and warping of the cutting wheel to adversely affect the dimensional accuracy of the magnet pieces as cut or sliced.
Furthermore, a serious difficulty is encountered when the thickness of the base wheel of an abrasive-bladed cutting wheel is decreased. As is shown in FIG. 1C illustrating an enlarged axial cross sectional view of the abrasive cutting blade 2 bonded to the outer periphery of a base wheel 1, it is usual that the thickness t2 of the abrasive blade is larger than the thickness t1 of the base wheel 1 so that, when the abrasive blade 2 cuts into a workpiece, a gap space called an xe2x80x9cescapexe2x80x9d having a thickness t3, which is usually in the range from 0.01 to 0.2 mm, is formed between the surface of the base wheel 1 and the workpiece under cutting. This escape serves as a discharge passage of cutting dust particles from the surface of the base wheel 1 which must be discharged as smoothly as possible by increasing the thickness t3 while the thickness t3 should be as small as possible in order to decrease the material loss caused by the cutting works.
Namely, the difficulty encountered in the use of a cutting wheel made from a base wheel 1 of a small thickness t1 is that the thickness t2 of the abrasive blade and, consequently, the thickness t3 of the escape cannot be large enough to ensure smoothness in the discharge of cutting dust and abrasive particles off the blade 2 so that the surface of the base wheel 1 is heavily scratched and damaged by the particles jammed in the gap space of the escape. This problem is particularly serious when the workpiece under machining is a rare earth-based sintered magnet block because a sintered rare earth magnet has a hardness equivalent to or even higher than that of the alloy tool steels conventionally used as a material of the base wheel in abrasive-bladed cutting wheels of the prior art along with remarkable brittleness resulting in heavy damages on the surface of the base wheel 1. Once the surface of the base wheel 1 has abrasive scratches with local plastic deformation, the base wheel 1 loses balance of stress between the two surfaces resulting in deformation of the base wheel 1 such as warping and undulation. This disadvantage is more serious when the thickness of the base wheel 1 is smaller to cause large warping or undulation even with very small scratches. Any small deformation in the base wheel 1 has an effect to further enlarge warping and undulation of the base wheel 1 by the stress in further cutting runs acting toward this effect so that the dimensional accuracy of the workpieces as cut or sliced is greatly decreased.
When the cutting work is conducted by using a multiple cutting wheel assembly illustrated in FIGS. 2A and 2B with a plurality of abrasive-bladed cutting wheels 6 as supported coaxially on a single shaft 10 keeping a space from the adjacent wheels 6 with spacers 3, in particular, the dimension, e.g., thickness, of the workpiece as cut or sliced is determined solely by the spacings between two adjacent cutting wheels so that any slightest warping or undulation of the base wheels 1 directly affects the dimensional accuracy of the products as cut or sliced unless the spacing between the cutting wheels 6 or, namely, the thickness of the spacers 3 is frequently adjusted resulting in a great decrease in the productivity of the cutting works or undue increase of the material loss by cutting.
The present invention accordingly has an object to provide a novel abrasive-bladed multiple cutting wheel assembly without the above described problems and disadvantages in the prior art assemblies of the similar type.
Thus, the abrasive-bladed multiple cutting wheel assembly provided by the invention is an assembly which comprises:
(A) a shaft for rotation;
(B) at least two abrasive-bladed cutting wheels each consisting of a base wheel having a center opening for insertion of the shaft, of which the diameter is not exceeding 200 mm and the thickness is in the range from 0.1 to 1 mm, and an abrasive blade layer bonded to the outer periphery of the base wheel, each cutting wheel being fixed on the shaft inserted into the center opening in the base wheel; and
(C) at least one spacer, the number of the spacers being smaller by one than the number of the cutting wheels, each having a center opening for insertion of the shaft and fixed on the shaft by inserting the shaft into the center opening at a position between two abrasive-bladed cutting wheels to define the spacing therebetween,
the base wheel of the abrasive-bladed cutting wheel being made from a cemented metal carbide and the abrasive blade layer of the abrasive-bladed cutting wheel being made from particles of an abrasive material bonded together with a bonding agent.
In particular, the base wheel made from a cemented metal carbide should have a Young""s modulus in the range from 45000 to 70000 kgf/mm2 or, alternatively, should have a Vickers hardness Hv in the range from 900 to 2000.
Further, the abrasive particles from which the blade layer is formed are preferably particles of diamond, particles of cubic boron nitride or a combination thereof having an average particle diameter in the range from 50 to 250 xcexcm and the volume fraction of the abrasive particles in the blade layer is preferably in the range from 10 to 50%, the balance being the bonding agent.